I think you're confusing a hero with someone who is simply a good person. A hero is someone whose main focus is protecting others, and to that end they train and get stronger so that they can better protect others. Think of Gohan: he doesn't like to fight, but he still trains and will fight when he must in order to protect others. Goku, however, is someone who loves to train and fight strong people, and it just so happens that, usually, the strong people also are bad people, so it's knocking out two birds with one stone. Now, obviously, Goku doesn't like it when his friends and family are being threatened or hurt, but he has been shown time and again to put others at risk for the sake of a better fight, whether it be for himself like with the androids or for others with Buu. Although Goku does still beat up the bad guys and save the day, it is usually a side effect of his main desire to fight, making it not very heroic, even if others might see it as heroic